In a prior proposal a gimbal mount was used to provide a supporting shaft with the appropriate freedom of movement. This was, seen to be unsatisfactory because the bearings would wear out very quickly and, even with repeated replacement of the bearings every few weeks, the apparatus had an average life span of about six months.
The wearing down of the bearings was attributed to two main causes, the first being that the motion of the apparatus required the bearings to rotate by angles rarely exceeding 15.degree., causing very uneven wear, and the second being that emery powder washed down from the grinding surface was able to get into the bearings, rapidly rendering them unusable.
Although the bearings can be shielded to a certain extent from the grinding powder by rubber sheets and washers such protection has never been completely effective and in any case makes replacement of the bearings more difficult.